Various methods for producing an α-hydroxyisobutyric acid amide by hydration of an acetone cyanohydrin in the presence of a catalyst composed mainly of manganese oxide have been disclosed. For example, Patent Document 1 discloses that in hydration of an acetone cyanohydrin using manganese oxide, reaction results are improved by adding acetone to a reaction raw material consisting of acetone cyanohydrin and water, and that in this case, the conversion of acetone cyanohydrin is 99.0% and the yield of α-hydroxyisobutyric acid amide is 95%. However, according to the method described in Patent Document 1, the catalyst life is not sufficiently improved, and it is difficult to carry out the method at large-scale commercial plants.
Several improved methods relative to the method described in Patent Document 1 have been proposed. For example, a method in which an oxidizing agent such as oxygen and ozone is allowed to coexist (Patent Document 2), a method in which pH of a reaction raw material is adjusted (Patent Documents 3 and 4), a method in which a portion of a reaction product liquid is circulated in order to adjust pH of a reaction raw material (Patent Document 3), a method in which carbon dioxide is allowed to coexist (Patent Document 5), a method in which a catalyst is pretreated with a reduction solution prior to the reaction (Patent Document 6), and a method in which the reaction is performed under reduced pressure (Patent Document 7) are disclosed.
These methods respectively exert effects of improving catalytic activity or catalyst life, but it is difficult to stably maintain a high acetone cyanohydrin conversion for a long period of time using a reaction raw material containing an acetone cyanohydrin at a concentration of 30% by weight or more. For example, Patent Document 4 describes a working example in which a method of adjusting pH of a reaction raw material was combined with a method of allowing an oxidizing agent to coexist and a reaction raw material containing an acetone cyanohydrin at a concentration of 30.4% by weight was used, but the life defined as the time for the conversion to be reduced to less than 50% of that at the start is not more than 58 days.
Further, Patent Documents 8 and 9 disclose a method in which, even when the conversion of acetone cyanohydrin is low, unreacted acetone cyanohydrin in a reaction product liquid is thermally decomposed into acetone and hydrocyanic acid, and these substances are separated from the reaction product liquid and collected, and then acetone cyanohydrin is made therefrom again. However, this method is not economical because extra energy is required for a thermal decomposition reaction and an acetone cyanohydrin synthesis reaction.